In the INVEST model, there are some problems with the annual water yield part.The annual water yield module runs successfully, but the effect of changing Z parameter on the result is negligible.The Z parameter is adjusted from 1.5 to 22.5, or even to 30, and the mean value of the output result changes slightly to six decimal places. In fact, the rule should not be so, the mean should vary greatly with the Z parameter (at least not so small), I am so confused and looking forward to your reply.
Do you see any significant variation in the result
output/per_pixel/fractp.tif? That is where the Z parameter is directly used.
Sorry for the late reply.The output of Z parameter 5 and 22.5 does not change at all in fractp.tif. The maximum, minimum, average, and standard deviation are the same.
I’m not sure if this explains things, but the User Guide says this about the Z parameter, and equation 2, the calculation it’s part of:
Z is an empirical constant, sometimes referred to as “seasonality factor”, which captures the local precipitation pattern and additional hydrogeological characteristics. It is positively correlated with N, the number of rain events per year. The 1.25 term is the minimum value of , which can be seen as a value for bare soil (when root depth is 0), as explained by Donohue et al. (2012). Following the literature (Yang et al., 2008; Donohue et al. 2012), values of ω (omega) are capped to a value of 5.
So perhaps it’s the case that no matter whether Z is set to 5 or 22.5, ω is maxed out, and capped to a value of 5, so the change in Z doesn’t have an effect?
So if you want to get accurate water yield output data, how do you solve this problem?